Hall effect devices are often used in sensor applications for contactless sensing of magnetic fields. Hall effect devices find widespread use in many automotive and industrial applications. For example, in automotive applications a Hall effect device may be used to measure wheel speed in an automatic braking system (ABS) speed sensor, by measuring the speed of a magnet. In such an example, if a magnet approaches a Hall sensor then the Hall sensor will measure an increase in the magnetic field, therefore allowing the speed of the wheel to be detected.
Hall effect devices are solid state electron devices that operate in response to a magnetic field based upon the Hall effect principle, a phenomenon by which a voltage differential is generated across an electrically conducting body in the presence of a magnetic field. Conventional Hall effect devices typically comprise a planar structure, known as a Hall plate, which is configured to generate an output signal (e.g., either voltage or current) that is proportional to an applied magnetic field.
Hall plates have orthogonal axes, such that applying a current along one of the orthogonal axes causes a voltage to be generated along another orthogonal axis in the presence of a magnetic field. Typically, a Hall plate is operated by injecting a current into a first input, grounding a spatially opposed second input on the same axes, and measuring a voltage between inputs of an orthogonal set of axes. For example, as shown in FIG. 1, a current 104 may be applied across a two dimensional conductive Hall plate 102. According to the Hall principle, the presence of a magnetic field B causes the negative charge carrying particles of the current 104 to vary their motion (according to the right hand rule as shown at 106) and generate an induced voltage differential between nodes V1 and V2, which is proportional to the magnetic field B.
The integration of Hall effect devices (e.g., Hall plates) into semiconductor bodies (e.g., silicon substrate) has become common in many applications. One main problem of Hall effect devices is the zero point offset/error, which is a non-zero output signal (e.g., voltage, current) provided by the Hall effect device in the absence of a magnetic field (i.e., magnetic field equal to zero). The offset of Hall effect devices is caused by small asymmetries of the device caused by manufacturing tolerances or mechanical stress or thermo-electric voltages. In order to reduce/remove the offset errors experienced by a Hall effect device, the Hall effect device may be configured to take readings along different orientations of the device. Such methods, known as “current spinning”, send current through a Hall effect device in different directions and combine the output signals in a manner which reduces the offset. For example, a Hall effect device may be rotated by 90° between measurements and then the average of the Hall output signals over a spinning cycle may be taken. While current spinning methods may reduce the offset errors (e.g. to 20 μT order of magnitude) such methods alone fail to completely remove the zero point error down to the noise level of 100 nT . . . 1 μT.
The cause of this residual zero point error is unclear. It can be proven that it must vanish for Hall effect devices with perfectly linear voltage-current-relationship. However, in modern CMOS technologies junction isolation techniques are used to isolate the Hall effect devices from other circuit elements on the same substrate. The width of the depletion layers associated with these reverse biased junctions depends on applied potentials and this leads to nonlinear current-voltage-characteristics of integrated Hall effect devices. The above stated small asymmetries of the Hall effect device are mixed up by the nonlinearity of the device and result in higher order offset error terms that cannot be eliminated by the spinning current principle. Therefore a method is sought to have a better control of the potentials inside the Hall effect device during a spinning current cycle.